Packaging of electronic devices provides mechanical support, protection of the electronic circuitry components, and a medium for interconnecting the chip to a circuit board for use in a system. As systems become smaller, a need exists to make packages as small as possible.
The packaging process begins with the fabrication of a crystalline semiconductor material, usually silicon or gallium arsenide. Individual dies (i.e., chips) are formed on the wafer at the same time. Then the wafer is separated into single dies. One limit in making small packages is the size of the dies (e.g., the size of the silicon).
Each of the individual dies is typically packaged in a chip carrier. External connections on the chip carrier package allow for the chip to be mounted on a printed wiring board. The chip carrier is electrically connected to the printed wiring board by surface mounting the chip carder directly to the mounting surface.
There are a variety of sizes for packages. For instance, thickness of the individual packages varies between different packages. For example, while some packages are only 1 mm think, other packages range up to 5 mm thick. Generally, for packages having leads, the smaller the number of leads that a package has, the thinner the package is itself. The disparity in package thicknesses is due to the use of more molding compound for packages having a greater number of leads, The added molding is required to hold the many leads rigid. Thinner packages do not have some of the reliability problems of larger packages.
The primary problem with larger packages is their susceptibility to cracking during the solder reflow process. The cracking results from moisture which has been absorbed by the plastic. During the solder reflow process, the package heats up to 240.degree.-260.degree. F., causing the absorbed water to turn to steam. The steam causes pressure to build up within the plastic, and the plastic cracks in order to relieve the pressure (i.e., the steam).
To prevent the cracking of the package, the package is protected from moisture. One method of protecting the package from the effects of moisture is to bake the packages at lower temperatures. By baking the package at a lower temperature, the moisture has an opportunity to be released by the plastic molding. This baking step is referred to as the preheat stage. Since the moisture is not trapped in the package, cracking is less likely. Also to prevent the further infiltration of moisture into the package, the chips are stored in moisture proof bags. The bags contain desiccant to keep the humidity within the bag to a minimum while the chips are being stored. The desiccant absorbs excess moisture. However, if the bag has been punctured or has been stored for too long a period of time, then the bag is likely to have absorbed moisture. Whether moisture has entered the storage bag is indicated by a color change in moisture sensitive material which is also included in the bag. Even with these precautions, the packages are baked gently upon removal from the bag before they are mounted on the board to insure against moisture and its effects. The performance of these procedures does not ensure that the packages are moisture free. Thus, even though these precautions are undertaken, damaged chips still may be produced.
When packages are thin, they are less likely to crack. Thin packages are less likely to crack because the packages are too thin to hold or contain a substantial amount of moisture and the preheat stage is enough to drive all of the moisture out of the package. Therefore, at the high temperature of the reflow stage, cracking does not occur because there is an insignificant amount of moisture remaining in the plastic molding.
Another problem associated with larger packages is that larger residual stresses are generated. In other words, large amounts of molding compound induce stresses in the die which the structure cannot support. The residual stresses occur in any molding operation and are accentuated by a mismatch of the thermal expansion between the leadframe, which is generally copper, the molding compound, and the silicon, all of which produce stresses and warpage. As a result, the package warps, resembling a "potato chip". Thus, it is desirable to make the package thin and small (in the x,y dimension) to decrease the die stresses.
As will be shown, the present invention comprises a thin small integrated circuit package. The package of the present invention comprises a thin molding compound locally around the die with a separate support for the leadframe to keep the leads in position.